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Advanced Colloids Experiment-Temperature-7 (ACE-T-7) - 12.06.17

Science Objectives for Everyone
The Advanced Colloids Experiment-Temperature-7 (ACE-T-7) experiment involves the design and assembly of complex three-dimensional structures from small particles suspended within a fluid medium. These so-called “self-assembled colloidal structures”, are vital to the design of advanced optical materials and active devices. In the microgravity environment, insight is provided into the relation between particle shape and interparticle interactions on assembly structure and dynamics: fundamental issues in condensed matter science.

Science Results for EveryoneInformation Pending

The following content was provided by Paul M. Chaikin, Ph.D., and is maintained in a database by the ISS Program Science Office.

Experiment Details

OpNom: ACE

Principal Investigator(s)
Paul M. Chaikin, Ph.D., New York University, New York, NY, United States

Co-Investigator(s)/Collaborator(s)
Andrew D. Hollingsworth, Ph.D., New York University, New York, NY, United StatesStefano Sacanna, Ph.D., New York University, New York, NY, United States

Developer(s)NASA Glenn Research Center, Cleveland, OH, United States ZIN Technologies Incorporated, Cleveland, OH, United States

New functional materials are, in principle, created using small particles suspended in fluid (called colloids) that self-organize into crystalline structures by means of entropic forces. The Advanced Colloids Experiment-Temperature-7 (ACE-T-7) experiments utilize confocal microscopy for time- and space-resolved, 3D imaging of non-spherical colloids whose phases can be controlled by adding depletants and/or adjusting spatial temperature changes around a particular location. The smaller particles allow the tuning of the interactions between the colloids, and in this way control the structure of the colloidal dispersion. The application of macroscopic thermal gradients allows the adjustment of particle concentration.

In ACE-T-7, the crystallization behavior of micron-sized colloidal cubes is studied by means of a tunable depletion interaction and a temperature-controlled sample cell. Under certain conditions, the particles should self-organize into crystals with simple cubic symmetry, which is set by the size of the nano-size depletant. Increasing particle number density using a temperature gradient, i.e., thermophoresis, induces formation of crystallites. Three-dimensional structures that are impossible to create or reform on earth due to sedimentation issues (high density contrast between particles and fluid) should be observed in microgravity.

Ultimately, the ability to design functional structures – based on micron-scale building blocks – with a variety of well-controlled three-dimensional bonding symmetries allow new devices for chemical energy production and storage, photonics and communication. Such materials might include photonic crystals with programmed distributions of defects. Optical technology utilizing such materials may offer intriguing solutions to unavoidable heat generation and bandwidth limitations facing the computer industry. The beginning of this process is understanding the basic interactions between micro- and nanoscale particles, and how to control them using external sources such as temperature and light.

Description

An outstanding problem in condensed matter science concerns the relation between particle shape, crystal symmetry and structure. The simplest and most symmetric crystal is cubic and is naturally comprised of cube-shaped ‘particles’. In atomic systems, these are cubic lattices of atoms; in our colloidal structures, the constituent particles are, in fact, colloidal cubes. Our research goal is to produce such colloidal structures, and study the dynamics of crystal nucleation and growth. Advanced Colloids Experiment-Temperature-7 (ACE-T-7) varies the size and concentration of the depletant in different samples with the goal of seeing the effect on crystallization. The imaging goal is to observe crystallites and resolve particle centroid positions with less than 20% error.

Due to their small gravitational height – related to the grossly mismatched density of the colloids and suspending media – the Brownian particles assemble in a quasi-2D fashion, growing only 2 or 3 particles in height. Larger gravitational heights might be achieved by dispersing the particles in a density matching solvent (e.g., an ethanol/bromoform mixture) to retard sedimentation for the 3D assembly. However, there are numerous difficulties associated with this type of experiment which often drastically changes the particle interactions.

Observing the particle interactions using confocal microscopy in microgravity eliminates these complicating factors, and increases the gravitational height by several orders of magnitude, thus allowing the possibility of making three dimensional structures. In this system, aggregation and complex structure formation is driven by anisotropic forces that cannot be generated in conventional sphere fluids. This allows additional control of the condensed phase where, for example, decreasing the size of the depletant locks neighboring sliding planes in registry and produces a true, simple cubic crystal.

By scanning the length of the thermal gradient cell containing the constituent particles, we should observe a continuous change in particle number density. Our previous microgravity experiments showed that a sample starting in the liquid state can be induced to crystallize at the cold end of such a cell. Confocal microscopy is key to the observation of these phenomena in real space and real time. Using this technique, the colloidal particles can be tracked spatially in three dimensions with great precision and over large time scales. Sample observation after mixing confirm homogenization of the samples.

Colloidal silica cubes crystallize in the presence of depletants. Depletion forces arise between colloidal particles that are suspended in a dilute suspension/solution of smaller particles/solutes. These depletants are preferentially excluded from the vicinity of the large particles. They can include polymers, molecular association structures (micelles), and – for the ACE-T-7 experiment – nanoparticles dispersed in the continuous phase. Fundamentally, the depletion force arises from an increase in osmotic pressure of the surrounding solution when colloidal particles get close enough such that the excluded depletants cannot fit in between them.

During nucleation and growth, changes in size density and distribution of critical nuclei, diffusion of particles to individual growing crystallites, density field in the surrounding fluid phase, instabilities (dendrites) of crystallites, healing of the dendrites and the ripening of the crystallites are expected. Diffusion of dislocations, annealing of grain boundaries and of stacking faults along with the time evolution of the crystal structure are also expected once the sample is crystallized.

Space Applications
Eventually, future space exploration may use self-assembly and self-replication to make materials and devices that can repair themselves. Self-assembly and evolutionarily optimized functional units are key to long-duration space voyages.

Earth Applications
This investigation involves several fundamental and practical aspects of soft matter science with potential applications on Earth. Self-assembly processes are crucial to making functional materials and devices from small particles. Improved design and assembly of structures in microgravity may have use in a variety of fields from medicine to electronics on Earth. Ultimately, the ability to design and build functional structures based on colloids allow new devices for chemical energy, communication, and photonics, including photonic materials to control and manipulate light.

Overall, one experiment (capillary cell) is conducted each week and lasts 1one to four days, depending upon the number of z-layers and longitudinal scans along the thermal gradient that are needed to determine the crystal properties. The experiment is repeated until all cell wells are tested. Sample modules are switched if air bubbles are too big. The rest of the week is used to analyze data (including cross-correlation using template matching algorithms to improve image resolution), re-write scripts, and adjust parameters. The number of capillaries per experiment is limited by data bottlenecks on IPSU (internet protocol setup utilities) and IOP (input/output operations per second). One capillary position is maintained. Alternatively, if this requirement takes an excessive amount of time, images can be registered in post-processing via port or stir bar location, or pattern of particles stuck to bottom of cover slip.

The general experimental protocol includes imaging and dynamics of single colloidal crystallites in each capillary. Samples are inspected before mixing the colloid using the in situ mixer. Next, XY offsets are defined and image particles are located. Then, bubble locations and possible primary/secondary Regions of Interest (ROI) are determined. Immersion oil from the auxiliary fluids container (AFC) is then applied. Scans of the same colloidal crystallite are repeated every 30 minutes. To obtain statistically meaningful data, at least ten colloidal crystallites in one capillary should be imaged. This calculates to at least 400 minutes for one capillary. Experiment is complete upon evaluation (estimate up to a three-day duration). If crystallites are growing, the protocol is repeated at 60-minute intervals to follow the kinetics.

For the temperature gradient cell experiments, the temperature controller is turned on and then researchers wait for the temperature gradient to equilibrate. The imaging goal is to observe crystallites and aggregates and resolve particle centroid positions with less than 20% error.